Reactive crystallization of ammonium polyvanadate from vanadium-bearing solution assisted by efficient ultrasound irradiation: Crystallization characteristics and growth process

Ultrasound crystallization (UC) technique was previously proposed to solve the issues caused by the acidic ammonium salts crystallization of vanadium. In this work, the influence of various parameters on vanadium crystallization characteristics were detailedly investigated. Results showed that high vanadium precipitation ratio (99.67%) and V2O5 purity (99.50%) of the final products can be obtained under the conditions of ultrasonic power of 600 W, initial pH value of 2.0, reaction temperature of 95 °C, ammonium salt addition coefficient of 0.5, reaction duration of 5 min and vanadium concentration of 20 g/L. Moreover, the NH4+ content in the supernatant liquid was notably reduced to 1.08 g/L. Crystallization kinetic analysis manifests that the crystallization rate constant augments with the increasing temperature, and the activation energy and pre-exponential factor is 71.87 kJ/mol and 7.44 × 109, respectively. Ultrasound enhances the collision frequency between NH4+ and vanadium ions, further improving the crystallization rate of the (NH4)2V6O16 crystals. Meanwhile, ultrasound induces a secondary nucleation, which would promote the intercalation among small flakes and further improves the nucleation and growth of the (NH4)2V6O16 crystals. Scanning electron microscopy analysis demonstrates that the flaky crystals are firstly formed, then a part of flakes gradually intercalate with each other, and finally most of the lamellar crystals complete the intercalation process. Meanwhile, selected area electron diffraction verifies the single crystalline characteristic of the micron-flakes, which grows along the (010) crystal plane. This work will lay theoretical and technical basis for UC technique appliance and promote the progress of the vanadium extraction industry.